Normally a hydraulic pump or motor returns about one-half to one gallon of oil per minute to the reservoir while in operation, which flow is referred to as the case drain flow. This normal operating loss of oil serves two purposes. The first is that of lubricating the moving parts of the pump or motor, there being no need for a separate source of lubrication for such a device when its own hydraulic oil is readily available. The second purpose is to serve as a heat exchange medium for removing excess heat from the pump or motor.
As the pump or motor is used and its parts gradually wear, the amount of such oil being lost gradually increases. Under normal operating conditions and without traumatic injury to the pump or motor an amount of wear will eventually occur which, if remedial steps are not taken, will eventually result in either inefficient operation or severe damage to the unit. Also in a complex hydraulic system, where many devices operate in conjunction with one or more motors and/or pumps, it is not always possible to know whether the source of trouble is in a motor or pump or one of the other various devices operating in the system.
When a problem persists in a hydraulic system, it is common to measure case drain flow across its various motors and pumps. Excessive flow would indicate that oil losses of a magnitude greater than one gallon per minute, or whatever is allowable, are occurring across the particular pump or motor, further indicating the need to repair or replace the unit.
The conventional method of measuring such an oil flow is to install an expensive flow test kit between the unit and the reservoir. In addition to the relatively complex test kit, other tools and specialized knowlege are required to install and monitor the unit. Oil flow is measured both at zero pressure and at normal operating pressure, from which measurements oil loss can be determined. Only in rare situations is it practical to permanently install such testing equipment in conjunction with a pump or motor. Due to the high cost of the oil test kit, its temporary installation, and the need for experienced personnel to take the readings, more often than not, the same is not accomplished until wear and/or damage to the pump or motor has progressed to a point calling for complete overhaul or replacement of the unit.